The invention relates generally to the field of active rectifier circuits, and more specifically to active rectifier circuits used in photovoltaic (PV) solar power systems as bypass rectifiers and blocking rectifiers.
FIG. 1 shows a high level diagram of an example PV solar array consisting of two strings 1 and 2 wired in parallel and feeding their combined currents into a converter 3. Each string consists of a blocking rectifier 4 and a plurality of solar panels 5, and each solar panel has an associated bypass rectifier 6. The purpose of the bypass rectifiers 6 is to protect the solar panels 5 from damage when the PV solar array is partially shaded. For example, in FIG. 1 one panel in the first string 1 is shaded by some obstruction 7 such as a cloud or tree branch, while the other panels all receive full sunlight. The unshaded panels try to force current to flow through the shaded panel, but the bypass rectifier 6 provides an alternate current path around the shaded panel, thus protecting it from damage. But since the first string 1 contains a shaded panel, its output voltage is lower than that of the second string 2 where all the panels are in full sunlight. The blocking rectifiers 4 prevent the string with the lower voltage 1 from draining current produced by the unshaded string 2.
The bypass and blocking rectifiers degrade the efficiency of the PV solar array because they dissipate significant amounts of power. As current flows through a rectifier in the forward direction (from anode to cathode) a voltage drop develops across the rectifier, resulting in heat being produced inside the rectifier. For example, a typical Schottky used as a bypass rectifier would have a forward voltage drop of about 500 mV at 10 A; thus wasting up to 5 W of power. Additionally, Schottky rectifiers have high reverse leakage currents, typically in the range of 10 mA when the junction temperature is 75° C., so energy is wasted in every bypass rectifier, even when none of the solar panels are shaded. A typical blocking rectifier has an even larger forward voltage drop of about 1.2V at 10 A, thus wasting up to 12 W each. What is more, power is wasted continuously in the blocking rectifiers as long as the sun is out, as opposed to the losses in the bypass rectifiers, which can vary as transient shading conditions come and go throughout the day.
The bypass and blocking rectifiers also reduce the systems reliability because of their high junction temperatures. The rectifiers are typically mounted inside enclosures to protect them from moisture and contamination, but these enclosures also have the undesirable effect of acting as thermal insulators that trap the heat. For example, a typical PV solar panel includes three bypass rectifiers inside a plastic enclosure mounted on the back side of the panel. If the entire panel is shaded, then all three bypass rectifiers can dissipate 5 W each, for a total of 15 W dissipation inside the small plastic enclosure. This can easily result in the bypass rectifiers junction temperatures exceeding 200° C., which can severely shorten their lifespans. What is more, Schottky bypass rectifiers are also vulnerable to a destructive phenomenon called thermal runaway because of their high leakage currents, which double for every 10° C. increase in junction temperature. As the leakage current increases, it heats the bypass rectifier further, which produces even more leakage current; this positive feedback loop sometimes leads to a the bypass rectifier failing and becoming a short circuit.
When just one bypass or blocking rectifier fails, it prevents the system from operating all the PV cells at their maximum power point. This can decrease the system efficiency by up to 50%. The large number of bypass and blocking rectifiers in a typical PV solar array makes their reliability all the more critical.
What is needed to increase the efficiency and reliability of PV solar arrays, are better rectifiers with lower forward voltage drop, lower reverse leakage current, and higher reliability. One solution well known to the art is an active rectifier, which consists of a transistor used as a switch, and circuitry for controlling the transistor such that the switch closes to allow current flow in the forward direction; and the switch opens to prevent current flow in the reverse direction. The forward voltage drop in an active rectifier can easily be ten times lower than in a Schottky rectifier at the same current, and twenty times lower than a silicon rectifier. Additionally, the reverse leakage current in an active rectifier is typically thousands of times lower than with a Schottky, thus eliminating the threat of thermal runaway. This not only boosts system efficiency at the beginning of life, but also helps to maintain peak efficiency over time because their dramatically lower junction temperatures make active rectifiers more reliable.
The main challenge faced when designing active rectifier circuits for PV solar arrays, is providing power to run the control circuitry. Most active rectifier circuits are used in the field of power conversion (e.g. switching-mode power supplies) where there is usually a supply voltage available to power the active rectifier control circuitry. In contrast, for bypass and blocking rectifiers in PV solar arrays, there is no readily available power source for the control circuitry; therefore, the active rectifier circuit must power itself.
To illustrate the problem of self-powering FIG. 2 shows an example of prior art disclosed in U.S. patent application Ser. No. 11/094,369 (the '369 application). The '369 application describes an active rectifier circuit that does not self-power, comprising: an anode 8; a cathode 9; a power MOSFET 10; an offset bias voltage source 11; and an operational amplifier (opamp) 12. Before the power MOSFET 10 is turned on, current can flow from anode 8 to cathode 9 via the body diode 14 that forms an integral part of the power MOSFET 10. The resulting voltage drop across the body diode 14 is larger than the bias voltage 11, so the differential voltage across the opamp 12 inputs is positive, causing the opamps output to swing high, which turns on the power MOSFET 10. Once the power MOSFET 10 turns on, the anode-to-cathode voltage drops drastically, but then negative feedback maintains the anode-to-cathode voltage at a constant level, equal to the offset bias voltage 11. The negative feedback mechanism operates as follows: the differential input voltage to the opamp 12 is the anode-to-cathode voltage minus the bias voltage 11; if the anode-to-cathode voltage falls below the offset bias voltage, the opamp 12 decreases the voltage applied to the gate of the MOSFET 10; the decreased gate voltage results in increased channel resistance in the MOSFET 10; the increased channel resistance results in increased anode-to-cathode voltage, bringing it back up to the level of the bias voltage 11, and thus closing the negative feedback loop.
While the circuit of FIG. 2 performs the basic functions of an active rectifier, it is not practical for applications such as bypass or blocking rectifiers in PV solar arrays because it is not self-powering and requires an external supply voltage 13 to power the opamp 12. For example, in FIG. 1, the potential between the bypass rectifier 6 at the top of the first string 1 and the bypass rectifier at the bottom of the same string can be hundreds of Volts. So, in order to utilize the circuit of FIG. 2 to replace the bypass rectifiers in FIG. 1, a power supply with many isolated outputs (one output for each active rectifier circuit) would be needed.
FIG. 3 shows another example of prior art disclosed in U.S. patent application Ser. No. 12/815,496 (the '496 application). The '496 application describes a self-powered active rectifier circuit that is in many ways analogous to the circuit from the '369 application: both circuits employ a power MOSFET 10 as the switch to conduct current from anode 8 to cathode 9. A charge pump 19 in FIG. 3 provides a large voltage gain similar to the opamp 12 in FIG. 2. The minimum voltage required to maintain operation of the charge pump 19 in FIG. 3 is roughly analogous to the offset bias voltage 11 in FIG. 2. Thus, both circuits employ similar feedback mechanisms to regulate the voltage from anode to cathode when current is flowing through the circuit in the forward direction. The main difference between the two inventions is that the circuit in FIG. 3 is self-powering because it is able to produce the power MOSFET gate drive voltage by multiplying the anode-to-cathode voltage with a charge pump 19. Therefore, an active rectifier like the one in FIG. 3 can replace the bypass rectifiers 6 in FIG. 1 without the need for any external power supplies.
But there are at least two drawbacks to the circuit in FIG. 3. First, it's poorly suited for use as a blocking rectifier because it's constructed as an Integrated Circuit (IC). A blocking rectifier typically must withstand at least 300V reverse bias, which is extremely difficult and costly to achieve with an IC. Second, the circuit is very complex; in order to get enough voltage gain, the charge pump 19 must have a large number of stages, each with a capacitor 15. What is more, the charge pump 19 requires a clock source 16, and a subcircuit 17 that produces several non-overlapping clock phases. And the charge pump 19 cannot start up on its own; it requires a start-up unit 18 consisting of yet more oscillators and charge pumps. All of this adds significantly to the cost and complexity of the circuit.
Accordingly, there is a continuing need in the field of photovoltaic solar power for an active rectifier circuit that is self-powered, has very low forward voltage drop and reverse leakage current, and is extremely reliable. The present invention fulfills these needs and provides other related advantages.